I want to select the text of all descendants of a node which *don't*
  have an attribute called 'id' (and none of the nodes which are
  descendants of nodes with id attributes).

By that I think you mean you want to select all text nodes that don't
have an ancestor with an id attribute, that would be

//text()[not(ancestor::*/@id)]

Note by the way that [@id = '']/doesn't test if te element has an id
attribute, just if it is empty <foo id=""/> woul dtest true, you want
[not(@id)] to test for the attribute itself rather than its value.

David


________________________________________________________________________
This e-mail has been scanned for all viruses by Star. The
service is powered by MessageLabs. For more information on a proactive
anti-virus service working around the clock, around the globe, visit:
http://www.star.net.uk
________________________________________________________________________